Fibronectin is a large glycoprotein containing around 5% carbohydrate. The characteristic form of plasma fibronectin is a disulfide-bonded dimer of 440,000 daltons, each subunit having a molecular weight of about 220,000 daltons. Normally found in plasma at a concentration of about 300 .mu.g/ml, fibronectin is extracted and purified using a method described by Hynes.sup.1. Plasma fibronectin is also known by various other names, including cold-insoluble globulin, anti-gelatin factor, cell attachment protein, cell spreading factor, and opsonic .alpha.2-surface binding glycoprotein. These names reflect biological activities of fibronectin such as cell recruitment, opsonization of particulate debris, and promotion of wound contraction. Reviews on structure and activities of fibronectin have been published elsewhere.sup.2,3.
Wound healing is usually divided into three phases: the inflammatory phase, the proliferative phase, and the remodeling phase. Fibronectin has been reported to be involved in each stage of the wound healing process, particularly by creating a scaffold to which the invading cells can adhere. Initially, many mediators, such as fibronectin and fibrinogen, are released to the wound site. Fibronectin promotes inflammatory cells migration into the wound and debris phagocytosis by the monocytes. Thereafter, angiogenesis and reepithelialization take place. At this stage fibronectin exerts chemotactic activity on endothelial cells, and promotes the migration of epithelial cells and fibroblasts onto the basal membrane. Fibronectin also appears to be an essential component of the remodeling phase where it plays a major role in the organization of collagen fibrils. The fibrillar collagen ultimately forms fibrous bundles that greatly enhance the tissue tensile strength, leading to wound closure.
Topically applied plasma fibronectin has been reported as being useful for increasing the rate of wound healing such as in corneal wounds.sup.4,5 and leg ulcers.sup.6. However, no one has described a suitable topical carrier for use in treating wounds that can ensure the delivery of an effective amount of fibronectin. A major limiting factor in developing an effective topical dosage form of a drug is not only having an active drug, but also having a formulation that allows the passage of the active drug from the carrier (cream, ointment, gel, etc.) into the site of delivery (which in the case of the present invention is a skin wound). Very active drugs, such as growth factors, may have no therapeutic value if the topical formulation does not allow the drug to move from the semi-solid carrier into the wound. Therefore, it would be highly desirable to develop a formulation which would maximize the contact time of the fibronectin with the wound and also control the release of fibronectin to the wound, thereby leading to high absorption values. The present invention provides such delivery system in the form of aqueous gels and a cream.